Flower color changes can be triggered by very small changes in soil pH and/or the the absence of metal ions in the soil. Metal ions are necessary to cement the color determining pigments together.
Light and temperature can also affect colors, but usually only to the extent that colors are more or less vibrant.

Columbines are noticeably susceptible to soil pH. I have a blue one that appeared in my garden several years ago and seedlings from it, just 5 ft. away are pink/salmon colored.

You might experiment with changing the soil pH slightly, by adding small amounts of lime (alkaline) or aluminum sulfate (acidic), and see if the original yellow color returns.
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Additionally most hybrids have difficulty maintaining their hybrid genetics and will start reverting to one of the parent plants. Some plants are more stable while others revert more quickly.